Well, that's because it searches in the... uh... hmmm. It looks like someone made changes to the source code but forgot to update the documentation. Here's the complete list of folders that are searched, direct from the source code:
static const char *bindirs[] = {
"/usr/bin",
"/usr/sbin",
"/usr/lib",
"/usr/lib64",
"/bin",
"/sbin",
"/etc",
"/usr/etc",
"/lib",
"/lib64",
"/usr/games",
"/usr/games/bin",
"/usr/games/lib",
"/usr/emacs/etc",
"/usr/lib/emacs/*/etc",
"/usr/TeX/bin",
"/usr/tex/bin",
"/usr/interviews/bin/LINUX",
"/usr/X11R6/bin",
"/usr/X386/bin",
"/usr/bin/X11",
"/usr/X11/bin",
"/usr/X11R5/bin",
"/usr/local/bin",
"/usr/local/sbin",
"/usr/local/etc",
"/usr/local/lib",
"/usr/local/games",
"/usr/local/games/bin",
"/usr/local/emacs/etc",
"/usr/local/TeX/bin",
"/usr/local/tex/bin",
"/usr/local/bin/X11",
"/usr/contrib",
"/usr/hosts",
"/usr/include",
"/usr/g++-include",
"/usr/ucb",
"/usr/old",
"/usr/new",
"/usr/local",
"/usr/libexec",
"/usr/share",
"/opt/*/bin",
NULL
};
static const char *mandirs[] = {
"/usr/man/*",
"/usr/share/man/*",
"/usr/X386/man/*",
"/usr/X11/man/*",
"/usr/TeX/man/*",
"/usr/interviews/man/mann",
NULL
};
static const char *srcdirs[] = {
"/usr/src/*",
"/usr/src/lib/libc/*",
"/usr/src/lib/libc/net/*",
"/usr/src/ucb/pascal",
"/usr/src/ucb/pascal/utilities",
"/usr/src/undoc",
NULL
};
Source